The invention relates to equipment used on aircraft to deliver compressed air from the power source, typically a turbine engine, to other air systems requiring a continuous supply of air such as environmental control systems, and more particularly to a system for controlling bleed air supplied by the engines of multi-engine aircraft in order to ensure a balanced supply of bleed air from each of the engines on the aircraft, thereby achieving balanced flow extraction.
With the exception of a limited number of smaller aircraft, most aircraft utilizing turbine engine propulsion units, both commercial and military, are powered by two or more turbine engines. Virtually all such aircraft divert bleed air from the engines to supply various other systems, most notably environmental control systems (ECS) which require a supply of conditioned air which is utilized for the crew and passenger environment, and in some instances electronic equipment onboard the aircraft is cooled by a portion of the bleed air.
It has been recognized for quite some time that in order to efficiency operate a multiple engine aircraft it is desirable to utilize bleed air supplied from all of the engines rather than from only one power plant. If, for example, the entire supply of bleed air to the aircraft is supplied by one engine, the result is significantly deteriorated overall aircraft fuel economy as well as excessive wear to the engine supplying the bleed air, since that engine also has to carry its share of the aircraft propulsion duties. Bleed air control systems have developed some degree of sophistication, and typically utilize a pressure regulating valve to control the amount of bleed air supplied from each engine to a heat exchanger used to cool the bleed air from that engine, with the cooled air from all the engines then being fed into a common manifold prior to distribution onboard the aircraft.
Aside from this rather significant accomplishment, most effort in the art of bleed air control systems has been directed toward preventing compressor surge in the various stages of the gas turbine engine by bleeding airflow from appropriate stages during certain flight conditions, with the excess air bled off typically being dumped into the ambient atmosphere. Two examples of such systems are U.S. Pat. No. 3,179,356 and 3,505,817, both of which are used to prevent back flow into lagging engines, and bleed flow to the atmosphere when there is excessive bleed air to prevent compressor surge. While in most cases acceptable performance has been achieved by existing systems, there remains one troubling problem plaguing bleed air control systems, with this problem posing significant (and expensive) instances of deteriorated system performance.
It will be recognized that pressure regulating valves are reasonably sophisticated mechanical devices, and as such are susceptible to some degree of mechanical tolerance inherent in the performance characteristics of such valves. Pressure regulating valves are designed to regulate bleed air pressure of air supplied from an engine to a heat exchanger, and typically operate with a tolerance of plus or minus 2.5 psig. While such a tolerance seems relatively good in view of the fact that this represents an error of approximately plus or minus 4%, it does nevertheless present a potentially highly damaging situation.
Using by way of an example an aircraft having two engines with both engines supplying bleed air, consider the case when one pressure regulating valve on the first engine has an error allowing pressure to be slightly larger than desired, for example 2 psig higher than desired, and the second pressure regulating valve on the other engine has an error in the opposite direction so that air leaving the second pressure regulating valve is 2 psig below the desired value. In this case, the pressure regulating valve supplying air at the higher pressure will supply significantly more air than the pressure regulating valve supplying air at the lower pressure, which means that one engine is supplying significantly more bleed air than the other engine.
While the situation thusly described is serious and has a detrimental effect on overall aircraft performance, an even more serious consequence is likely to occur when the difference in supply pressure is more than minimal. If there is a difference of several psig in air supplied by two pressure regulating valves, as in the example just described, the result can be that the higher pressure regulating valve will force air backwards in the system in the wrong direction through the lower pressure regulating valve, a situation which is known as the higher regulator swamping out the lower regulator. When this occurs, one engine will supply all of the bleed air required by the aircraft and the other engine will supply no bleed air at all, since the regulating valves are designed to prevent reverse flow of air therethrough by closing.
The results of pressure regulating valve error are present to some degree even with fairly small amounts of error in the pressure regulating valves, with the results of such imbalance increasing in severity with the magnitude of the error. The most immediately noticeable result is substantially diminished aircraft fuel economy, since the engine supplying significantly more bleed air will burn more fuel than the other engine saves. It will be appreciated that such a reduction in fuel economy will have severe economic results, particularly in the case of commercial aircraft.
A second and even more expensive result of the problem is an increased level of engine distress. The engine required to supply substantially more bleed air will wear out significantly faster, since the blades of the turbine engine will be running hotter due to the increased amount of bleed air tapped off due to the engine pressure regulating valve supplying air at a higher pressure than the other engine(s) pressure regulating valves. This results in the requirement that the engine be rebuilt or replaced at a substantially earlier time and with substantially fewer hours on the engine.
While the previous discussion has concentrated on the example of a twin engine aircraft, it is apparent that the pressure regulation problem will be at least as severe on aircraft having more than two engines. For example, if a four engine aircraft has one pressure regulating valve supplying air at higher pressure than the other three, it is possible for that pressure regulating valve to swamp out the other three regulating valves, thereby requiring a single engine to supply all the bleed air utilized by the aircraft. The results in this case are an unacceptably serious reduction in both fuel economy and engine wear characteristics. It is therefore seen that a system to ensure more accurate pressure regulation of bleed air is highly desireable since such a system would result both in better aircraft economy and a lower degree of mechanical wear in the engine. Such a system, whether utilized on a two engine aircraft or an aircraft having more than two engines will likely pay for itself in a relatively short operating time, and therefore represents a highly desireable addition to any bleed air control system.